354 Scientific Geology. 



markably pure and I may add, remarkably regular and continuous in 

 its slaty structure. (Nos. 806 to 816.) 



4. Quartz and Talc. In this variety the talc is usually scaly, and 

 the quartz arenaceous. Sometimes, however, the latter is coarsely 

 granular, or hyaline. When the talc predominates, and the quartz is 

 in minute grains, they form the whetstone slate. (Nos. 817 to 825.) 



In Smithfield, Rhode Island, this rock is extensively quarried for 

 whetstones. I learn from Dr. Webb, that the number annually quar- 

 ried at that place and sent away, can hardly be less than 5000 dozen. 

 Indeed, so important has this manufactory become, that the General 

 Assembly of Rhode Island have passed an act to regulate the inspec- 

 tion and sale of the whetstones. It is not true, as stated by Mr. 

 Eaton, that all our whetstone quarries are in talcose slate: for those 

 in Norwich and Enfield are in decided mica slate. 



This rock is employed, also, as a substitute for fire bricks in the 

 lining of furnaces. 



5. Quartz, Talc, and Mica. This variety may be considered, 

 either as mica slate, which takes into its composition more or less of 

 talc, or as talcose slate, containing mica. Probably but little of our 

 talcose slate can be found, that does not embrace a small proportion of 

 mica : but talc and mica often resemble each other so exceedingly, 

 that it is very difficult to say whether the rock is talcose or mica 

 slate. I have felt this difficulty most in relation to a considerable part 

 of the slaty rock in Berkshire county. And although I have there 

 marked no talcose slate, yet I hardly expect that all geologists will 

 follow my example. (Nos. 826 to 831.) 



The 4th and 5th varieties constitute the greater part of the talcose 

 slate formation. 



6. Talc and Carbonate of Lime. Sometimes talcose slate lies next 

 to limestone as in Whitingham, Vt. and near the junction of the two 

 minerals, they a re mixed together. But the variety is hardly worth 

 naming, (No. 832.) 



7. Talc, Quartz, and Carbonate of Iron. It might be more pro- 

 per, perhaps, to describe the carbonate of iron as disseminated 

 through the talcose slate ; though the iron most commonly occurs in 

 the variety, No. 4. And this mineral, by its decomposition, imparts 

 a character to this rock which will be noticed every where. It 

 abounds in spots of the color of iron rust, and this is particularly 

 the case where masses of quartz exist of considerable size. If I mis- 



